In the continuation of our studies on the stereochemistry of enzyme reactions the major focus will be on the steric course of various reactions of one-carbon metabolism and on the mechanism of deoxyhexose formation, with some effort still devoted to the completion of ongoing studies on pyridoxal phosphate enzymes and on the isoprenylation of tryptophan in Claviceps. The studies on one-carbon metabolism will examine stereochemical aspects of DNA methylation, methionine biosynthesis, the formation of the methylenedioxy function and the berberine bridge in alkaloid biosynthesis, and the double methylation involved in thienamycin biosynthesis. In the work on deoxyhexose formation we will study stereochemical and mechanistic aspects of the formation of CDP-3,6-dideoxyglucose from CDP-4-keto-6-deoxyglucose by a pyridoxamine phosphate-dependent enzyme from Salmonella. Exploratory studies will also be carried out on the biosynthesis of 2,6-dideoxyhexoses in Streptomyces and on the stereochemistry of thiamine pyrophosphate-catalyzed enzyme reactions.